Recovery boilers, also called recovery furnaces, are used to reclaim chemicals used in, for example, the paper making process. The boiler is surrounded by a series of cooling tubes, which are occasionally separated to define an air port into the body of the boiler, for introduction of air to assist in the combustion taking place in the center of the boiler. Normally there are three principal air port types, primary, secondary and tertiary air ports (although there can be others, as some recovery boilers have quarternary air ports, for example). The primary air ports are typically smaller and are more numerous, disposed on the walls of the furnace firebox near the bottom of the furnace. Air supplied to the primary ports is at a relatively low pressure, and provides combustion air primarily to the perimeter of the char bed in the interior of the furnace. Adjustment of the primary air port air allows control of the shape and position of the char bed's perimeter. Secondary air ports are typically larger and fewer in number than primary air ports, and are usually placed around the walls of the firebox higher up than the primary air ports, but below the liquor spray nozzles that spray in the fuel, called liquor, to be evaporated, gasified, pyrolized, oxidized and reduced. Air through the secondary ports is normally at a higher pressure than is the primary air and is used to control the position of the top of the char bed as well as to aid in combustion of gasses rising from the char bed. Tertiary air ports are located above the liquor spray nozzles and are generally fewer in number than secondary air ports, and usually employ a still higher pressure air to promote combustion and mixing of gasses rising in the firebox.
For use in conjunction with the air ports of some style boilers, air port castings have been developed, to define the frame of the opening of the air port, as well as providing some protection to the tubes against damage by any automatic port rodding devices or by manual port rodding, since the ports must frequently be freed of any built up excrescent material by rodding, repeated insertion of a cleaning rod into the air port for dislodging of built up material, to ensure adequate air flow into the boiler. Other boiler styles employ a nozzle that may be welded directly to the tubes around the air port opening, to provide protection against rodding and perhaps a more directed flow of air into the boiler.
In addition to the casting/nozzle distinction at the air ports, there are two predominant style of recovery boiler tube designs, those employing adjacent cooling tubes 10 (which may be, for example, three inches in diameter) with a membrane 12 (which may be one inch wide, for example) between adjacent tubes (see FIG. 1) or so called tangent tube designs, wherein, for example, the tubes are two inch diameter and are one thirty-second of an inch apart. In this tangent tube design, for example, as manufactured by Combustion Engineering until the late 1960s, the tubes are welded together, the weld occupying the 1/32nd inch space. Other examples of tangent tube designs exist, with 3 inch tubes on 3 inch centers, by Babcock and Wilcox (until late 1960s) and by Gotaverken (manufactured until 1991).
To form the air port with the membrane style boiler, which can be a boiler manufactured by Combustion Engineering (after 1967), for example, referring to FIG. 1, the membrane 12a, 12b is eliminated at either "outer" side 14, 16 of two adjacent tubes 10a, 10b and the tubes are bent outwardly and away from each other, to form an opening 18 which defines the air port. In nozzle design boilers, as the tubes flare outwardly, the membrane widens, up to two inches wide at 20, and then ends, forming a crotch plate 22, the area of transition from one inch membrane to two inch width port opening. Because of thermal stress and heat transfer problems, the membrane is restricted to about a one inch maximum width on newer boilers, otherwise, the membrane will corrode, or even worse, will develop a crack. On older boilers, a membrane up to 2 inches wide was often used. Because of this width, the crotch plate does not get enough cooling and tends to develop corrosion. Also, because of its location at the interior of the boiler, the crotch plates on primary level ports are potentially subject to contact by the smelt bed, resulting in even more severe thermal gradients and further corrosion. Therefore, the crotch plate is a potential failure zone; if a crack develops at the crotch plate, there is a risk that the crack will migrate to one of the adjacent tubes, which are welded to the membrane, potentially allowing cooling water to escape. In such a case, there may be a smelt-water reaction between the cooling water in the tubes and the molten smelt in the recovery boiler. The resulting explosion can kill or injure nearby workers, as well as potentially destroying the recovery boiler. Therefore, the crotch plate is carefully inspected from the interior of the furnace during shut down and any cracks located are ground down and the membrane or crotch plate is repaired. The required grinding raises further issues, since whenever grinding is performed near a tube, there is a risk of nicking the tube. If the tube is nicked, it then must be repaired by certified welders, including being x-rayed to inspect for damage, adding to the repair expense. In general, it is very costly and time consuming to make the required inspections and repairs, such inspection and repair often being the critical path of shut down, determining how long the shut down will last.
The welded nozzle boiler designs are difficult to maintain, since any maintenance or changes at the nozzles run into the potential for nicking or otherwise damaging the cooling tubes with the corrective actions required as noted above. Also, cracks in the crotch plate may be concealed by the nozzle or its weldment.
It is desirable to install a port damper assembly (see, for example U.S. Pat. No 5,307,745, entitled REMOVABLE DAMPER FOR CHEMICAL RECOVERY FURNACE) to allow selective adjustment of the size of the opening of the air port, to alter the pressure or volume of air flowing into the boiler. Installing a port damper assembly, and raising the air pressure in the windbox, can help reduce the situation where the smelt bed in the interior of the recovery boiler moves and contacts the tubes (or membrane in that style of boiler). The windbox is a duct or other sealed chamber that is pressurized to force air into the furnace through the air ports. However, in order to install a port damper assembly, heretofore, it has been necessary to install a damper guide along the length of the air port and/or extending above it, to provide a flat guide for the guillotine style damper as it is raised up away from the air port. Also, it is necessary to provide a flat surface at the face of the casting in boilers of such design, to operate as a sealing surface for the damper. Modern castings are curved because of the out-of-plane tube bends used to define the air port. Therefore, modification of the furnace air port area is required. A flat built up area is added, but due to limited space, the flat surface is typically relatively far away from the air port opening (there is not room close to the port to provide the structure for the flat surface, and, the windbox may be such that there is not sufficient space above the air port to accommodate the vertical travel of the port damper assembly). Thus, the damper is moved farther back away from the air port opening where there is more room, reducing the effectiveness of the damper. To maximize damper effectiveness, the damper should operate as close as possible to the tube opening so the air jet expansion/dissipation occurs after (not prior to) exiting the port opening. The above noted modifications add to the concerns and expense of mounting air port damper assemblies, and the modifications to provide the flat surface add more repair consideration which must be handled. In order to remove and replace the port casting, it becomes necessary to dismantle the damper guide, as the damper guide mounting hardware or the guide itself would not need to be replaced but may restrict access to the bolts holding the port casting to the boiler, further complicating the maintenance procedures.
In prior out-of-plane air port casting assemblies, mounting of the casting to the port is accomplished by upper, lower, left and right bolts which are mounted (e.g. welded) to the outer side of a junction (typically to the membrane) between two adjacent cooling tubes above, below, to the left and to the right of the air port. After the casting is positioned and tightened down on the bolts, a refractory material is applied around the lower bolt area, to provide protection against the harsh environment in which the lower bolt and windbox exist. Since the refractory material hardens to a cement like consistency, later removal of the casting is difficult, presenting the so called "bottom bolt" problem, as the refractory material must be chiseled away and the threads and nut securing the casting are likely damaged, further complicating removal. Also, the threads of the other mounting bolts can be damaged, sometimes requiring that the bolts be cut off in order to remove the casting.